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2.a.Q.What Is Switching? Ex-plainthe three Switching techniques with diagrams. Ans. A network is a set of connected devices.

Whenever we have multiple devices, we have the problem of how to connect them to make one-to-one communication possible. One solution is to make connection by meshtopology or startopology etc.These methods, however, are impractical and wasteful when applied to very large networks.The number and length of the links require too much infrastructure to be cost-efficient.A bettersolution is switching. A switched network consists of a series of interlinked nodes,called switches. Switches are devices capable of creating temporary connections between two or more devices linked to the switch. In a switched network, some of these nodes areconnected to the end systems (computers or telephones, for example).Others are usedonly for routing. Example:

We can divide today's networks into three broad categories: circuit-switched networks,packet -switched networks, and message-switched. Packet-switched networks can further be divided into two subcategoriesvirtual-circuit networks and datagram networks as shown below

CIRCUIT-SWITCHED NETWORKS : A circuit-switched network consists of a set of switches connected by physical links. A connection between two stations is a dedicated path made of one or more links. However, each connection uses only one dedicated channel on each link. Each link is normally divided into n channels by using FDM or TDM.In circuit switching, the resources need to be reserved during the setup phase; the esources remain dedicated for the entire duration of data transfer until the teardown phase.

Three Phases
The actual communication in a circuit-switched network requires three phases: connection setup, data transfer, and connection teardown. >Setup Phase Before the two parties (or multiple parties in a conference call) can communicate, a dedicated circuit (combination of channels in links) needs to be established. The end systems are normally connected through dedicated lines to the switches, so connection setup means creating dedicated channels between the switches. >Data Transfer Phase After the establishment of the dedicated circuit (channels), the two parties can transfer data. >Teardown Phase When one of the parties needs to disconnect, a signal is sent to each switch to release the resources.

If the message is going to pass through a packet-switched network. no matter what is the source or destination. 4. there are setup and teardown phases in additionto the data transfer phase. For example. while a circuit-switched network is implemented in the physical layer and a datagram network in the network layer. As in a datagram network.or on demand. The reader may ask how the intermediate switches know where to send the packet if there is no final destination address carried by a packet.DATAGRAM NETWORKS:
In data communications. As in a circuit-switched network. When a switch receives a packet. all packets follow the same path established duringthe connection. 5. and there is no scheduled processing time for each packet. the packet must wait if there are other packets being processed. we might have to wait. 2. 3. if we do not have a reservation at a restaurant. __________________________
------------------------------------VIRTUAL-CIRCUIT NETWORKS: A virtual-circuit network is a cross between a circuit-switched
network and a datagram network. The size of the packet is determined by the network and the governing protocol. Resources are allocated on demand. this lack of reservation may create delay.
. first-served basis. as in a circuit-switched network. as in a datagram network. It has some characteristics of both. But this may change in the future. As with other systems in our daily life.The answer will be clear when we discuss virtual-circuit identifiers in the next section. we need to send messages from one end system to another. This means that there is no reserved bandwidth on the links. the address in the header has local jurisdiction (it defines what should be the next switch and the channel on which the packet is being canied). Resources can be allocated during the setup phase. A virtual-circuit network is normally implemented in the data link layer. The allocation is done on a firstcome. it needs to be divided into packets of fixed or variable size. However. As in a circuit-switched network. not end-to-end jurisdiction. data are packetized and each packet carries an address in the header. 1. In packet switching. there is no resource allocation for a packet.

Both protocols are used and are very similar. if a station that is holding the token fails.2. If one of the links in the main ring fails. The token must be monitored to ensure it has not been lost or destroyed. When the station has no more data to send. Note that for this
. The second ring is for emergencies only (such as a spare tire for a car). it inserts a token in an empty frame (this may consist of simply changing a 0 to a 1 in the token bit part of the frame) and inserts a message and a destination identifier in the frame. the stations in a network are organized in a logical
ring. Empty information frames are continuously circulated on the ring. 4. Ans: Token Ring: In the token-passing method. it releases the token. When a computer has a message to send. Explain the Token ring network ( IEEE 802·5 ) & FDDI. The Token Ring protocol is the second most widely-used protocol on local area networks after Ethernet.or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time. It removes the message from the frame. Stations must be limited in the time they can
have possession of the token.
The token scheme can also be used with bus topology LANs.The standard for the Token Ring protocol is Institute of Electrical and Electronics Engineers (IEEE) 802. there is a predecessor and a successor. After the failed link is restored. 2. The right to this access has been passed from the predecessor to the current station. And finally. 5. The right will be passed to the successor when the current station has no more data to send. The IBM Token Ring protocol led to a standard version. the successor is the next one in line. If the workstation sees that it is the destination for the message.Q.5. Another function of token management is to assign priorities to the stations and to the types of data being transmitted. the auxiliary ring becomes idle again.5. This means that the token does not have to have the address of the next successor. the token cannot be seen by other stations. The current station is the one that is accessing the channel now. 3. for each station.But how is the right to access the channel passed from one station to another? In this method. The frame continues to circulate as an "empty" frame. a special packet called a token circulates through the ring.the token will disappear from the network. the system automatically combines the two rings to form a temporary ring. The IEEE 802.In other words. When the frame gets back to the originator. The frame is then examined by each successive workstation. For example. When a station has some data to send.b. Very briefly. passing it to the next logical station in the ring.The dual ring topology uses a second (auxiliary) ring which operates in the reverse direction compared with the main ring. it copies the message from the frame and changes the token back to 0. In the physical ring topology. the whole system fails. it sees that the token has been changed to 0 and that the message has been copied and received. The predecessor is the station which is logically before the station in the ring. The Fiber Distributed-Data Interface (FDDI) also uses a Token Ring protocol. here is how it works: 1.
FDDI : Token management is needed for this access method. The problem with this topology is that if one of the links-the medium between two adjacent stations fails .It then holds the token and sends its data.
A Token Ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a bit.5 Token Ring technology provides for data transfer rates of either 4 or 16 megabits per second. it waits until it receives the token from its predecessor. ready to be taken by a workstation when it has a message to send. the successor is the station which is after the station in the ring. token management is needed to make low-priority stations release the token to highpriority stations. specified as IEEE 802. when a station sends the token to its successor. The possession of the token gives the station the right to access the channel and send its data.

Q. or printer) has its own network interface card (NIC). The high-speed Token Ring networks called FDDI (Fiber Distributed Data Interface) and CDDI (Copper Distributed Data Interface) use this topology. The NIC fits inside the station and provides the station with a 6-byte physical address. the source address is a unicast address. to be more exact). multicast and broadcast addresses? Ans: Unicasting : In unicast communication. in the IP datagram. The destination address. (i) one bit per signal component 3000bits/second=3000 signal component/second Frequency = 3000hz=3khz (i i) three bits per signal component 3000bits/second=(3000/3) signal component/second=1000 signal component/second
. or broadcast. extending capacity to 200 Mbps. it forwards the packet through only one of its interfaces (the one belonging to the optimum path) as defined in the routing table. each station needs to have two transmitter ports and two receiver ports.a. The relationship between
the source and the destination is one-to-one. The primary ring offers up to 100 Mbps capacity. what frequency is necessary to support a bit rate of 3 kbps using (i) one bit per signal component (i i) three bits per signal component? Ans:3kbps=3000bits/second.
3. Multicasting: In multicast communication. an FDDI local area network can support thousands of users. otherwise. there is one source and a group of destinations. multicast. but the destination address is a group address. In this type of communication. A broadcast destination address is forty-eight Is. the relationship between the sender and the receiver is one-to-one. nonnally written in hexadecimal notation. workstation. both the source and destination addresses. a dual ring can extend 100 km (62 miles). FDDI is frequently used on the backbone for a wide area network (WAN). The FDDI protocol is based on the Token Ring protocol. Broadcasting:In broadcast communication. 2>A unicast destination address defines only one recipient. with a colon between the bytes. In this type of communication. the relationship between the sender and the receivers is one-to-many. There is only one source.The broadcast address is a special case of the multicast address. A multicast destination address defines a group of addresses. the relationship between the source and the destination is one-to-all. The Internet does not explicitly support broadcasting because of the huge amount of traffic it would create and because of the bandwidth it would need.c.topology to work. Addressing : Each station on an Ethernet network (such as a PC. Note that in unicasting. What are the differences between unicast. the address is unicast. when a router receives a packet. it can also carry data. 06:01 :02:01:2C:4B
6 bytes =12 hex digits =48 bits
Unicast. The router may discard the packet if it cannot find the destination address in its routing table. Imagine the traffic generated in the Internet if one person wanted to send a message to everyone else connected to the Internet. and Broadcast: Addresses A source address is always a unicast address-the frame comes from only one station.Q.
2. If the secondary ring is not needed for backup. however. there is one source and one destination. The Ethernet address is 6 bytes(48 bits). can be unicast. but all the other hosts are the destinations. the recipients are all the stations on the LAN. which defines one or more destinations. 1>If the least significant bit of the first byte in a destination address is 0. it is multicast. According to Nyquist. The single ring can extend the maximum distance. Multicast. are the unicast addresses assigned to the hosts (or host interfaces. In addition to being large geographically. FDDI (Fiber Distributed Data Interface) is a set of ANSI and ISO standards for data transmission on fiber optic lines in a local area network (LAN) that can extend in range up to 200 km (124 miles).The group address identifies the members of the group. The relationship is one-to-many.

b.7900 Hz (iii) Three signal sources (iv) A 200-Hz guard band between each signal source.The frequency of the modulated signal is constant for the duration of one signal element. We use the first carrier if the data element is 0. find the maximum bandwidth for each signal
source:
(i)FDM multiplexing (ii) Total available bandwidth . Both peak amplitude and phase remain constant for all signal elements. we use the second if the data element is 1. Binary FSK (BFSK):One way to think about binary FSK (or BFSK) is to consider two carrier frequencies.Frequency = 1000hz=1khz 3.Q. we have selected two carrier frequencies.
3x5
Ans: a)Frequency Shift Keying:In frequency shift keying. Explain the following terms with examples:
a) FSK b) AM c) PSK.but changes for the next signal element if the data element changes.f1 and f2. and the difference between them is very small. Ans:{7900-(200X2)}Hz=7500Hz (7500/3)Hz=2500Hz 2500Hz 200Hz 2500Hz 200Hz 2500Hz
4. the frequency of the carrier signal is varied to
represent data.Q.
. Given the following information. In Figure.Normally the carrierfrequencies are very high.

Although the carrier signal is only one simple sine wave. the middle of one bandwidth is f1 and the middle of the other is f2. Both f1 and f2 are apart from the midpoint between the two bands. each with a different amplitude.As Figure shows. Again the carrier signals are only simple sine waves. which combines ASK and PSK. c) Phase Shift Keying: In phase shift keying. it is the phase. However. and the other with a phase of 180°. PSK is superior to FSK because we do not need two carrier signals. PSK is less susceptible to noise than ASK.9 gives a conceptual view of PSK. the criterion for bit detection is the amplitude of the signal. In other words. one with a phase of 0°. Today. ASK is normally implemented using only two levels. Figure 5. the process of modulation produces a nonperiodic composite signal. Both frequency and phase remain constant while the amplitude changes.in PSK. The peak amplitude of one signallevel is 0. The difference between the two frequencies is Bandwidth for BFSK Figure also shows the bandwidth of FSK. the phase of the carrier is varied to represent two or more different signal elements. each with its own carrier frequency Cil or f2). Binary ASK (BASK) Although we can have several levels (kinds) of signal elements. PSK is more common than ASK or FSK. we have chosen a value greater than (l + d)S. This is referred to as binary amplitude shift keying or on-off keying (OOK). However. where S is the signal rate and the B is the bandwidth. The value of d is between 0 and 1. there is normally another factor involved. in which we have only two signal elements. we will see that QAM. We can think of FSK as two ASK signals. b) Amplitude Shift Keying: In amplitude shift keying. then the required bandwidth is B=(l+d)xS+ What should be the minimum value of ? In Figure.
. Both peak amplitude and frequency remain constant as the phase changes.
Binary PSK (BPSK)
The simplest PSK is binary PSK. which depends on the modulation and filtering process. the other is the same as the amplitude of the carrier frequency. Figure gives a conceptual view of binary ASK. the amplitude of the carrier signal is varied to create signal elements.
Bandwidth for ASK Figure also shows the bandwidth for ASK. It can be shown that the minimum value should be at least S for the proper operation of modulation and demodulation. B =(1 +d) x S The formula shows that the required bandwidth has a minimum value of 5 and a maximum value of 25. In ASK. If the difference between the two frequencies is . is the dominant method of digital to analog modulation. but the modulation creates a nonperiodic composite signal with continuous frequencies. Noise can change the amplitude easier than it can change the phase. This means that the bandwidth can be expressed as shown. called d. Binary PSK is as simple as binary ASK with one big advantage-it is less susceptible to noise.

The purpose of the OSI model is to show how to facilitate communication between different systems without requiring changes to the logic of the underlying hardware and software. An open system is a set of protocols that allows any two different systems to communicate regardless of their underlying architecture. No bandwidth is wasted for separating two carrier signals. The physical layer data
consists of a stream of bits (sequence of Os or 1s) with no interpretation. It also defines the procedures and functions that physical devices and interfaces have to perform for transmission to Occur.Q Explain with a suitable diagram. it is a model for understanding and designing a network architecture that is flexible. The physical layer is also concerned with the following:
oThe physical layer defines the characteristics of the
interface between the devices and the transmission medium.
o Representation of bits.ISO is the organization.Bandwidth Figure also shows the bandwidth for BPSK. the International Standards Organization (ISO) is a multinational body dedicated to
worldwide agreement on international standards. robust. 5.a.
9
Ans: Established in 1947. The bandwidth is the same as that for binary ASK. It deals with the mechanical and electrical specifications of the interface and transmission medium. To be transmitted.the working of OS1reference model.
.
Seven layers of the OSI model:
Physical Layer:
The physical layer coordinates the functions required to carry a bit stream over a physical medium. The physical layer defines the type of encoding (how Os and I s are changed to signals). and interoperable. OSI is the model. bits must be encoded into signals--electrical or optical. The OSI model is not a protocol. It also defines the type of transmission edium. but less than that for BFSK. The reason is that the signal element with phase 180° can be seen as the complement of the signal element with phase 0°.Implementation The implementation of BPSK is as simple as that for ASK. It was first introduced in the late 1970s. An ISO standard that covers all aspects of network communications is the Open Systems Interconnection model. This gives us a clue on how to implement BPSK.

___________________________________________________________________________________ The physical layer is responsible for movements of individual bits from one hop (node) to the next. the sender and the receiver clocks must be synchronized. When two or more devices are connected to the same link.o Data rate. or a hybrid topology (this is a combination of two or more topologies). • Flow control. half-duplex. forming a ring). data link layer protocols are necessary to determine which device has control over the link at any given time. • Physical addressing. a star topology (devices are connected through a central device). Whereas the data link layer oversees the delivery of the packet between two systems on the same network (links). a ring topology (each device is connected to the next. the data link layer imposes a flow control mechanism to avoid overwhelming the receiver. the physical layer defines the duration of a bit. Error control is normally achieved through a trailer added to the end of the frame. The simplex mode is a one-way communication. The data link layer divides the stream of bits received from the network layer into manageable data units called frames.If two systems are connected to the same link. • Error control. In a pointto-point configuration.
o Line configuration. only one device can send. The physical layer also defines the direction of transmission between two devices:
simplex. It makes the physical layer appear error-free to the upper layer (network layer). the other can only receive. ______________________________________________________________________________________ _ The data link layer is responsible for moving frames from one hop (node) to the next. The sender and receiver not only must use the same bit rate but also must be
synchronized at the bit level. to a reliable link. If frames are to be distributed to different systems on the network. The physical layer is concerned with the connection of devices to the media. The data link layer adds reliability to the physical layer by adding mechanisms to detect and retransmit damaged or lost frames. possibly across multiple networks (links). but not at the same time. two devices can send and receive. In a multipoint configuration. a raw transmission facility. there is usually no need for a network layer.
. which is how long it lasts. The transmission rate-the number of bits sent each second-is also defined by the physical layer. two devices can send and receive at the same time. It also uses a mechanism to recognize duplicate frames. two devices are connected through a dedicated link. If the frame is intended for a system outside the sender's network. the data link layer adds a header to the frame to define the sender and/or receiver of the frame. or full-duplex.6 shows the relationship of the data link layer to the network and physicallayers.
Data Link Layer
The data link layer transforms the physical layer.
o Physical topology.
Network Layer
The network layer is responsible for the source-to-destination delivery of a packet. In simplex mode. a link is shared among several devices. In the half-duplex mode. Figure 2. In a full-duplex (or simply duplex) mode. Devices
can be connected by using a mesh topology (every device is connected to every other device). the network layer ensures that each packet gets from its point of origin to its final destination. the receiver address is the address of the device that connects the network to the next one. In
other words. a bus topology (every device is on a common link). If the rate at which the data are absorbed by the receiver is less than the rate at which data are produced in the sender.Other responsibilities of the data link layer include the following:
•
Framing. The physical topology defines how devices are connected to make a network. In other words.
o Transmission mode. • Access control.
o Synchronization of bits.

A message is divided into transmittable segments. on the other hand. The transport layer header must therefore include a type of address called a service-point address (or port address). A connecti onoriented transport layer makes a connection with the transport layer at the destination machine first before delivering the packets. Like the data link layer. source-todestination delivery means delivery not only from one computer to the next but also from a specific process (running program) on one computer to a specific process (running program) on the other. After all the data are transferred. The network layer gets each packet to the correct computer. _________________________________________________________________________________________ _ The transport layer is responsible for the delivery of a message from one process to another. the transport layer is responsible for flow control.However. Computers often run several programs at the same time. The transport layer. For this reason. the connection is terminated. the transport layer is responsible for error control. with each segment
containing a sequence number. Other responsibilities of the network layer include the following: • Logical addressing. The sending transport layer makes sure that the entire message arrives at the receiving transport layer without error (damage. if the two systems are attached to different networks (links) with connecting devices between the networks (links). However. These numbers enable the transport layer to reassemble the message correctly upon arriving at the destination and to identify and replace packets that were lost in transmission. When independent networks or links are connected to create intemetworks (network of networks) or a large network. The network layer adds a header to the packet coming from the upper layer that.
o Segmentation and reassembly. or duplication). the connecting devices (called routers or switches) route or switch the packets to their final destination. overseeing both error control and flow control at the source-to-destination level. If a packet passes the network boundary. among other things. we need another addressing system to help distinguish the source and destination systems.
o Flow control. Error correction is usually achieved through retransmission.
o Connection control. A
connectionless transport layer treats each segment as an independent packet and delivers it to the transport layer at the destination machine. Like the data link layer. it does not recognize any relationship between those packets. The physical addressing implemented by the data link layer handles the addressing problem locally. ensures that the whole message arrives intact and in order. flow
control at this layer is performed end to end rather than across a single link.
Transport Layer
The transport layer is responsible for process-to-process delivery of the entire message. We discuss logical addresses later in this chapter. _________________________________________________________________________________________ _ The network layer is responsible for the delivery of individual packets from the source host to the destination host. includes the logical addresses of the sender and receiver. One of the functions of the network layer is to provide this mechanism. loss. The transport layer can be either connectionless or connecti onoriented. error
control at this layer is performed process-to process rather than across a single link. • Routing. Whereas the network layer oversees source-to-destination delivery of individual packets. the transport layer gets the entire message to the correct process on that computer. Other responsibilities of the transport layer include the following:
o Service-point addressing. as though each piece belonged to a separate message.
Session Layer
. there is often a need for the network layer to accomplish source-to-destination delivery.A process is an application program running on a host. However. whether or not it does. It treats each one independently.
o Error control.

shared database management. The infonnation must be changed to bit streams before being transmitted. For example. to retrieve files from a remote computer for use in the local computer. To carry sensitive information. the only pages that need to be resent after system recovery are pages 501 to 523. talks to the host. The user's computer talks to the software terminal which.
o Compression. Because different computers use different encoding systems. and so on. a system must be able to ensure privacy. Pages previous to 501 need not be resent. Encryption means that the
sender transforms the original information to another form and sends the resulting message out over the network. whether human or software. It establishes. Decryption reverses the original process to transform the message back to its original form. it is advisable to insert checkpoints after every 100 pages to ensure that each 100-page unit is received and acknowledged independently. and management. and video. the presentation layer is responsible for interoperability between these different encoding methods. to a
stream of data. The session layer allows a process to add checkpoints.
o Encryption. This application allows a user to access files in a remote host (to
make changes or read data). data link. and network) are not sufficient for some processes. Specific services provided by the application layer include the following:
o Network virtual terminal. compression. and to manage or control files in a remote computer locally. numbers. if a system is sending a file of 2000 pages. remote file access and transfer. or synchronization points. To do so. The presentation layer at the sender changes the information from its sender-dependent format into a common format. and encryption. It allows the communication
between two processes to take place in either halfduplex (one way at a time) or full-duplex (two ways at a time) mode.
Presentation Layer
The presentation layer is concerned with the syntax and semantics of the information exchanged between two systems.
Application Layer
The application layer enables the user. It provides user interfaces and support for services such as electronic mail. if a crash happens during the transmission of page 523.
o File transfer. The session layer allows two systems to enter into a dialog. and it
allows a user to log on to a remote host. and other types of distributed information services. and vice versa. Data compression
becomes particularly important in the transmission of multimedia such as text. A network virtual terminal is a software version of a physical terminal. access. Specific responsibilities of the presentation layer include the following:
o Translation. The session layer is the network dialog controller. _________________________________________________________________________________________ _ The session layer is responsible for dialog control and synchronization.Specific responsibilities of the session layer include the following:
o Dialog control. In this case. the application creates a software emulation of a terminal at the remote host. The presentation layer at the receiving machine changes the common format into its receiver-dependent format.
o Mail services. and synchronizes the interaction among communicating systems. This application provides the basis for e-mail forwarding and storage.
.The services provided by the first three layers (physical.
o Synchronization. The remote host believes it is communicating with one of its own terminals and allows the user to log on. audio. The processes (running programs) in two systems are usually exchanging information in the form of
character strings. to access the network. in turn. _________________________________________________________________________________________ _ The presentation layer is responsible for translation. maintains. Data compression reduces the number of bits contained in the information.

links a workgroup of task-related computers. but today can also be used for high-speed data connection to the Internet. Switches are devices capable of creating temporary connections between two or more
. building.25. Depending on the needs of an organization and the type of technology used..and video information over large geographic areas that may comprise a country. LANs are designed to allow resources to be shared between personal computers or workstations. We normally refer to the first as a switched WAN and to the second as a point-to-point WAN. we
have the problem of how to connect them to make one-to-one communication possible.called switches. Ans:
a) Switch :A network is a set of connected devices. One solution is to make connection by meshtopology or startopology etc. Currently. normally to the Internet. software (e. The resources to be shared can include hardware (e. however. A common example of a LAN.The number and length of the links require too much infrastructure to be cost-efficient. _________________________________________________________________________________________ _ The application layer is responsible for providing services to the user. audio. 5.b.
Ans:
Local Area Network A local area network (LAN) is usually privately owned and links the devices in a single office.A bettersolution is switching. This type of WAN is often used to provide Internet access. Whenever we have multiple devices. are impractical and wasteful when applied to very large networks. Another example is the cable TV network that originally was designed for cable TV. An early example of a switched WAN is X. The point-topoint WAN is normally a line leased from a telephone or cable TV provider that connects a home computer or a small LAN to an Internet service provider (lSP). 5. which usually comprise a router (internetworking connecting device) that connects to another LAN or WAN. It is designed for customers who need a high-speed connectivity. or campus (see Figure 1. and have endpoints spread over a city or part of city.g. or it can extend throughout a company and include audio and video peripherals. A good example of a MAN is the part of the telephone company network that can provide a high-speed DSL line to the customer. In Chapters 17 and 18 we discuss wide-area networks in greater detail. LAN size is limited to a few kilometers.b. found in many business environments. for example.or even the whole world. an application program). image. a network designed to provide connectivity between end users. a printer).10).o Directory services.Q Describe the features of the following devices : a) Switch b) Gateway c) Repeater. A WAN can be as complex as the backbones that connect the Internet or as simple as a dial-up line that connects a home computer to the Internet.. A switched network consists of a series of interlinked nodes. a LAN can be as simple as two PCs and a printer in someone's home office. or data. a continent. Wide Area Network A wide area network (WAN) provides long-distance transmission of data.g. This application provides distributed database sources and access for global information
about various objects and services.Q What do you understand by the terms – i) LAN ii) MAN iii) WAN? Give examples for each. engineering workstations or accounting PCs. Metropolitan Area Networks A metropolitan area network (MAN) is a network with a size between a LAN and a WAN.These methods. It normally covers the area inside a town or a city. The switched WAN connects the end systems.

A repeater does not actually connect two LANs.
c) Repeater:
A repeater is a device that operates only in the physical layer. and a switch. it connects two segments of the same LAN. some of these nodes areconnected to the end systems (computers or telephones. A gateway is normally a computer that operates in all five layers of the Internet or seven layers of OSI model. Both the computers of Internet users and the computers that serve pages to users are host nodes. a node or stopping point can be either a gateway node or a host (end-point) node. The gateway is used to filter unwanted application-layer messages. To extend this
. A repeater can overcome the 10Base5 Ethernet length restriction. Signals that carry information within a network can travel a fixed distance before attenuation endangers the integrity of the data. which knows where to direct a given packet of data that arrives at the gateway. The repeater then sends the refreshed signal. Example:
We can divide today's networks into three broad categories: circuit-switched networks.the length of the cable is limited to 500 m. In the network for an enterprise. for example).packet -switched networks. which furnishes the actual path in and out of the gateway for a given packet. move it up to the OSI application layer.a network designed to use the OSI model can be connected to another network using the Internet model.
A gateway is a network point that acts as an entrance to another network. In this standard. Packet-switched networks can further be divided into two subcategoriesvirtual-circuit networks and datagram networks as shown below
b)Gateway:
Although some textbooks use the terms gateway and router interchangeably. reads it.Others are usedonly for routing. This means that it can be used as a connecting device between two internetworks that use different models. In a switched network. A repeater can extend the physical length of a LAN. most of the literature distinguishes between the two. regenerates the original bit pattern. The gateway connecting the two systems can take a frame as it arrives from the first system. and remove the message. On the Internet. and interprets it. For example. A repeater is not a device that can connect two LANs of different protocols. A repeater receives a signal and. Gateways can provide security. a computer server acting as a gateway node is often also acting as a proxy server and a firewall server. The segments connected are still part of one single LAN. A gateway is often associated with both a router. A gateway takes an application message. and message-switched.devices linked to the switch. The computers that control traffic within your company's network or at your local Internet service provider (ISP) are gateway nodes. before it becomes too weak or corrupted.

25. Carrier sense multiple access (CSMA) requires that each station first listen to the medium (or check the state of the medium) before sending. it regenerates and forwards it to the other port. increase the performance.25 protocol:
Frame Relay is a virtual-circuit wide-area network that was designed in response to demands for a new type ofWAN in the late 1980s and early 1990s. X.25 is still being used by the Internet. Originally X. but it is being replaced by other WANs. The Internet.25 packet. the frame transmission time must be at least two times the maximum propagation time. and abort the transmission.25 has several drawbacks: a. called a datagram. And X. X. the device begins to transmit its first
. but it cannot eliminate it.
Ans:
a) X.An amplifier cannot discriminate between the intended signal and noise. Each device senses whether the line is idle and therefore available to be used. has its own network layer. it amplifies equally everything fed into it. The chance of collision can be reduced if a station senses the medium before trying to use it. A repeater forwards every frame.This is so because the station.25 protocol b)CSMA CD c)Circuit switched and packet switched networks. but the portions of the network separated by repeaters are called segments.
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The CSMA method does not specify the procedure following a collision. A repeater does not amplify the signal. Therefore.length.25 has its own network layer. does not keep a copy of the frame and does not monitor the line for collision detection. The repeater acts as a two-port node. X.
6. (i) For CSMAlCD to work. if any. Flow and error control at both layers create a large overhead and slow down transmissions. CSMA is based on the principle "sense before transmit" or "listen before talk.
b)CSMA CD:
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To minimize the chance of collision and. X. there was a need for higherdata-rate WANs.25 was designed in the 1970s. we divide the cable into segments and install repeaters between segments. to X."CSMA can reduce the possibility of collision. not an amplifier. the CSMA method was developed.25 has a low 64-kbps data rate. it regenerates the signal.A repeater is a regenerator. b. Carrier sense multiple access with collision detection (CSMA/CD) augments the algorithm to handle the collision. Before sending the last bit of the frame. X. By the 1990s. (ii) Carrier Sense Multiple Access/Collision Detect (CSMA/CD) is the protocol for carrier transmission access in Ethernet networks. When it receives a weakened or corrupted signal. when the available transmission media were more prone to errors. some organizations were using a virtual-circuit switching network called X. once the entire frame is sent. used X. On Ethernet. but operates only in the physical layer. This was so because X.25 for encapsulation in the X. This doubles the overhead. For example. In other words. c. it creates a copy. any device can try to send a frame at any time. not for the Internet. which means if the Internet wants to use X.25 requires acknowledgments for both data link layer frames and network layer packets that are sent between nodes and between source and destination. at the original strength. If it is. This means that the user's data are encapsulated in the network layer packets of X.25. therefore.25 that performed switching at the network layer. When it receives a frame from any of the ports. The possibility of collision still exists because of propagation delay.25 was designed for private lise. the Internet must deliver its network layer packet. Note that the whole network is still considered one LAN. however. which needs wide-area networks to carry its packets from one place to another.Q Write short notes on the following: a) X. bit for bit. However. Prior to Frame Relay. the sending station must detect a collision. we need a restriction on the frame size.25 has extensive flow and error control at both the data link layer and the network layer. it has no filtering capability.25.the Internet.

Each device then waits a random amount of time and retries until successful in getting its transmission sent.3 standard. If another device has tried to send at the same time.
.frame. CSMA/CD is specified in the IEEE 802. a collision is said to occur and the frames are discarded.